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## Description
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Hello 👋 this PR introduces the second stage of changes to support [ADR-038](https://github.com/cosmos/cosmos-sdk/pull/8012) state listening. This is rebased on top of the [first segment](https://github.com/cosmos/cosmos-sdk/pull/8551), which introduces the low level changes to the MultiStore and KVStore interfaces and implementations, the new WriteListener types, and the new listen.KVStore type.
In this segment we introduce the StreamingService interface, an implementation that writes out to files, and it's integration and configuration at the BaseApp level.
The idea was to have the first segment reviewed independently first but if people think it is easier/more time efficient to review both at the same time then we could start here.
Thanks!
This work is towards satisfying [ADR-038](https://github.com/cosmos/cosmos-sdk/blob/master/docs/architecture/adr-038-state-listening.md)
---
Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.
- [x] Targeted PR against correct branch (see [CONTRIBUTING.md](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#pr-targeting))
- [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [x] Code follows the [module structure standards](https://github.com/cosmos/cosmos-sdk/blob/master/docs/building-modules/structure.md).
- [x] Wrote unit and integration [tests](https://github.com/cosmos/cosmos-sdk/blob/master/CONTRIBUTING.md#testing)
- [x] Updated relevant documentation (`docs/`) or specification (`x/<module>/spec/`)
- [x] Added relevant `godoc` [comments](https://blog.golang.org/godoc-documenting-go-code).
- [x] Added a relevant changelog entry to the `Unreleased` section in `CHANGELOG.md`
- [x] Re-reviewed `Files changed` in the Github PR explorer
- [x] Review `Codecov Report` in the comment section below once CI passes
Ian Norden